Backport PR #1193: Fix get_loader returning None when load_jupyter_server_extension is not found (#1193) (#1199)Co-authored-by: Steven Silvester <steven.silvester@ieee.org> Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

jupyter_server/extension/utils.py

@@ -26,19 +26,24 @@ def get_loader(obj, logger=None):
     underscore prefix.
     """
     try:
-        func = getattr(obj, "_load_jupyter_server_extension")  # noqa B009
+        return getattr(obj, "_load_jupyter_server_extension")  # noqa B009
     except AttributeError:
-        func = getattr(obj, "load_jupyter_server_extension", None)
-        warnings.warn(
-            "A `_load_jupyter_server_extension` function was not "
-            "found in {name!s}. Instead, a `load_jupyter_server_extension` "
-            "function was found and will be used for now. This function "
-            "name will be deprecated in future releases "
-            "of Jupyter Server.".format(name=obj),
-            DeprecationWarning,
-        )
-    except Exception:
-        raise ExtensionLoadingError("_load_jupyter_server_extension function was not found.")
+        pass
+
+    try:
+        func = getattr(obj, "load_jupyter_server_extension")  # noqa B009
+    except AttributeError:
+        msg = "_load_jupyter_server_extension function was not found."
+        raise ExtensionLoadingError(msg) from None
+
+    warnings.warn(
+        "A `_load_jupyter_server_extension` function was not "
+        "found in {name!s}. Instead, a `load_jupyter_server_extension` "
+        "function was found and will be used for now. This function "
+        "name will be deprecated in future releases "
+        "of Jupyter Server.".format(name=obj),
+        DeprecationWarning,
+    )
     return func
 
 

tests/extension/mockextensions/mockext_deprecated.py

@@ -0,0 +1,12 @@
+"""A mock extension named `mockext_py` for testing purposes.
+"""
+# Function that makes these extensions discoverable
+# by the test functions.
+
+
+def _jupyter_server_extension_paths():
+    return [{"module": "tests.extension.mockextensions.mockext_deprecated"}]
+
+
+def load_jupyter_server_extension(serverapp):
+    pass

tests/extension/test_utils.py

@@ -1,6 +1,11 @@
 import pytest
 
-from jupyter_server.extension.utils import validate_extension
+from jupyter_server.extension.utils import (
+    ExtensionLoadingError,
+    get_loader,
+    validate_extension,
+)
+from tests.extension.mockextensions import mockext_deprecated, mockext_sys
 
 # Use ServerApps environment because it monkeypatches
 # jupyter_core.paths and provides a config directory
@@ -17,3 +22,10 @@ def test_validate_extension():
     assert validate_extension("tests.extension.mockextensions.mockext_user")
     # enabled at Python
     assert validate_extension("tests.extension.mockextensions.mockext_py")
+
+def test_get_loader():
+    assert get_loader(mockext_sys) == mockext_sys._load_jupyter_server_extension
+    with pytest.deprecated_call():
+        assert get_loader(mockext_deprecated) == mockext_deprecated.load_jupyter_server_extension
+    with pytest.raises(ExtensionLoadingError):
+        get_loader(object())